Curves for Parallel and series pumps operation 1

[extremechanical]

Hello Forum,

I am told, when pumps are operated in Parallel, performance curves must match as closely as possible, while in series curves dont have to match! This is something I need to understand. is there a reason for this statement. Any explanation please.


Thanks a lot!

 

[BigInch]

If your idea is not to match flowrates, or balance power loading of each pump, there is no real reason you cannot do it. It depends on your system curve and what you're trying to do as to how well it will work or not. If both pumps can find a balance point against your system curve they will each settle in to thier individual operating points. It just might not be the one you expect. You have three elements in the system that will all have to balance flows and pressures at the junction, pump 1, pump 2 and the downstream piping. The pressure will have to be equal at the point where both pumps discharge flows meet the downstream pipe inlet. If each pump has the capability to flow some amount at that pressure, the pressure required to move that flowrate down the downstream piping, the system flowrate will be the sum of the two pump's individual flowrates at that common discharge pressure.

Independent events are seldomly independent.

 

[Artisi]

This link will or should answer all your questions.

http://www.lawrencepumps.com/Newsletter/news_v07_i4_Apr10.html

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[extremechanical]

Thanks both.Artisi/ Biginch, I did understand your explanation for pumps in series....but Why performance curves of pumps in series dont have to match?

 

[BigInch]

Both pumps are always at the same flowrate. Nothing you or anybody else can do to change that.
The only possibility is that both head curves add together a each flowrate. However they will settle out at a pump flowrate equal to what your system curve shows that it can deliver for a pressure equal to the sum of the two pump's heads.

Independent events are seldomly independent.

 

[Artisi]

BigInch: think you mean same head not same flow rate.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[LittleInch]

extremechanical,

Although there are notable exceptions as noted above, in most circumstances, pumps in parallel have the same suction and discharge conditions. Therefore if you don't closely match the performence curves, one pump will end up doing a lot more work than the other and hence you won't get the flow you want from your system. It is difficult to get it precise and if you look at the amps going into "identical" pumps in a parralel configuration, you'll always find one doing more than the others, but hopefully not more than 10 to 15%.

For series pumps, flowrate is identical, but the added head is dependant on each pump and the steepness or variablility of your system curve or discharge pressure. E.g if your normal discharge pressure into a system or vessel was 50 bar then you would size pump A to do the duty. If the pressure in your vessel or system then went up to 70 bar for some operating reason, then you could add pump B with a differential pressure of 20bar to do the duty, making sure that the suction side of the second pump and its seals can handle the high inlet pressure. So long as both your pumps can handel the same flowrate, then the curves can vary, but the design flowrate should be the same or similar.

Think of this like a wagon train - If you have two horses side by side and one is a cart horse and the other a pony, you'll find it difficult to go in straight line (parallel pumping)and one will do much more work than the other - two "identical" cart horses will work much better. If you string the pony in front of the cart-horse you will get traction from both of them (series pumping). A little odd to think of it that way, but it might help to get the picture across.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Artisi]

BigInch. Sorry misread the earlier post re series pumping, still thinking parallel.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[extremechanical]

Little inch, Great concept, great explanation , though you term it "odd" with the pony and horse tales (!), I liked it!!

Great , thanks!

Biginch, thanks for revising the issue!